Hotfix 1.9.1 fixed the bug allowing the new drain valves to drain intake air this guide shows how to revert it back (also yes i used the windows snip tool to highlight deal with it).
How to Revert Drainable Air Glitch Back (Step-by-Step)
Step 1. Getting to Your Game Location
Right click kerbal space program on your library and click properties.
Click the local files tab on the new window and click "browse local game files".
Step 2. Navigating the Game Files
If you followed the previous step you should be in a window similar to this:
Click the gamedata folder.
Next click squad.
You should now be in a folder with a few .cfg files the one we want is ResourcesGeneric.cfg
Step 3. Editing the Config File
Before continuing: I recommend you copy the ResourcesGeneric.cfg file and saving it to a different folder.
Open the ResourcesGeneric.cfg file with notepad.
Next push control+F to open the searchbar type air into the text box and push enter.
You should be brought to the intake air section of the config we want the bit highlighted above.
Replace the highlighted text with the following.
isDrainable = true
showDrainFX = true
drainFXPriority = 7
drainForceISP = 5
drainFXDefinition = gasDraining
It should now look exactly like this screenshot make sure the brackets are the same as the screenshot after this just save the config in the original folder.
Congrats you have re enabled air draining.